Brief noxious heat pulses (2.0 sec duration; peak temperature equals 51.5 degrees C) programmed by a computer and generated by a contact thermode have been used to study peripheral and central nervous system mechanisms of first and second pain. Previous research has demonstrated that heat-induced first and second pains are related to impulse conduction in A delta heat nociceptive afferents and C polymodal afferents respectively. When 4 identical heat pulses are applied to the same location on the hand, first pain decreases in perceived intensity while second pain summates. The latter is related to CNS mechanisms since the summation still occurs when the location of the thermode changes between each heat pulse. Work in progress is focused on spinal cord dorsal horn mechanisms of summation of second pain. Spinothalamic tract neurons and interneurons of the dorsal horn, which receive input from A delta and C nociceptive afferents, are studied. Data are analyzed with the aid of a computer system. BIBLIOGRAPHIC REFERENCES: Price, D.D.: Modulation of first and second pain by peripheral stimulation and by psychological set. In: J.J. Bonica and D. Albe-Fessard (Eds.), Advances in Pain Research and Therapy, Vol. 1, Raven Press, New York, 427-432, 1977. Price, D.D., Hu, J.W., Dubner, R., and Gracely, R.H.: Peripheral Suppression of first pain and central summation of second pain evoked by noxious heat pulses. Pain 3:57-68, 1977.